Place the chocolate chunks and unsalted butter in a heat resistant bowl over simmering water. Stir constantly for the chocolate to melt evenly.
Once the chocolate has melted, remove the chocolate from the heat. Add the Grand Marnier, espresso powder, and salt. Mix well until fully incorporated.
Optional: If deciding to use the egg whites, whip the egg whites on high-speed with a mixer until stiff peaks start to form. Add the cream of tartar and whisk for another couple minutes. Gradually add the sugar until fully incorporated. Gently fold the egg whites in with the chocolate.
Mix on high-speed the whipping cream until it begins to thicken. Gradually add the powdered sugar, vanilla, and salt. Mix until firm and fluffy. Remember to not overmix the whipped cream, or it will become chunky. Remove 1½ cups of the whipped cream and set aside. (This will be used to decorate the chocolate mousse later).
Gently fold the remaining whipped cream into the chocolate mixture until fully combined and fluffy.
If transferring the chocolate mousse into glasses, put the chocolate mousse in a piping bag or a Ziploc bag with the corner cut off. Pipe the chocolate mousse out of the bag into the glass in a swirl motion. Cover and chill in the refrigerator for at least 2 hours for the flavor to develop and set. Or if serving from the bowl, refrigerate and cover the bowl.
Before serving the chocolate mousse, dollop the 1½ cups of whipped cream evenly on top of the glasses or pipe the whipped cream in a design you like. Garnish the chocolate mousse with berries and mint leaves. Serve cold. Enjoy!